No tf data. Actual error: Fixed Frame [base_link] does not exist

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Hi,

I try to make a URDF in ROS Melodic on Kubuntu 18.04. I made a small URDF with one link to test if it works, but I can't open it with RViz. I get a Global Status: Warning -> Fixed Frame -> No tf data. Actual error: Fixed Frame [base_link] does not exist

The onle link in my URDF is called "base_link" so I do not see where the problem is.

My URDF file looks like this:

<?xml version="1.0"?>
<robot name="tank_robot">

    <link name="base_link">
        <pose>0 0 0.1 0 0 0</pose>
        <visual>
            <geometry>
            <cylinder length="0.1" radius="0.2"/>
            </geometry>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <material name="black">
                <color rgba="0 0 0 1"/>
            </material>
        </visual>
    </link>
</robot>

The file is called "tank.urdf".

The launch file with which I open RViz and load the urdf looks like this:

  <launch>
      <arg
        name="model" />
      <arg
        name="gui"
        default="False" />
      <param
        name="robot_description"
        textfile="$(find tank_robot)/urdf/tank.urdf" />
      <param
        name="use_gui"
        value="$(arg gui)" />
      <node
        name="joint_state_publisher"
        pkg="joint_state_publisher"
        type="joint_state_publisher" />
      <node
        name="robot_state_publisher"
        pkg="robot_state_publisher"
        type="state_publisher" />
      <node
        name="rviz"
        pkg="rviz"
        type="rviz"
        args="-d $(find tank_robot)/rviz/urdf.rviz" />
  </launch>

Can someone help me to understand what is going wrong?